One well-known process for retreading tires comprises the technique of adhesively attaching a pre-cured tread onto a suitably prepared tire casing by means of a layer of appropriate bonding material, and of then subjecting the tire and tread assembly to heat and pressure so as to bond the tread permanently to the casing. Another prior art process comprises placing the casing in an appropriate curing chamber, and vulcanizing a tread of uncured rubber directly onto the periphery of the casing.
In carrying out the retreading technique by which a pre-cured tread is bonded to a tire casing, the casing is first buffed to remove as much of the old tread as possible. A strip of pre-cured tread is then cemented around the periphery of the buffed tire casing, and the assembly is covered by a thin envelope composed of a rubber-like material. A special inner tube, known as curing bag, may then be inserted into the interior of the tire casing to retain pressure within the tire during the curing process or, in some instances, pressure may be introduced directly into the casing itself. A special inner rim is mounted in place between the beads of the tire so that pressure can be maintained inside the tire during the curing process.
The assembly is then placed into a lower section of a mold, and the upper section of the mold is placed over the assembly to provide and enclosed curing chamber, the two sections being held together by an appropriate locking ring. Pressurized air may then be introduced into the curing bag within the tire casing in the chamber, or pressurized steam may be introduced directly into the tire casing; and pressurized steam is then introduced into the annular space between the tread/casing combination and the inner peripheral surface of the curing chamber to create the pressure and temperature necessary to bond the tread permanently to the tire casing by vulcanizing action.
As the steam pressure outside of the tread/casing combination in the curing chamber is increased, the pressure within the casing should similarly be increased to prevent the tire from being bulged inwardly, and thereby destroying the bond between the tread and the peripheral surface of the tire casing. As the pressure outside of the tire is increased to exceed normal atmospheric pressure, the pressure inside the tire may be similarly increased without any adverse effect on the tire. The apparatus of the invention assures that a predetermined pressure differential between the inside and outside pressures will be retained, and it automatically maintains the predetermined pressure differential throughout the process, so as to avoid damage to the casing, or destruction of the bond between the casing and the tread, as could occur should the pressure differential exceed certain limits.
Specifically, the present invention in one of its aspects is concerned with improved apparatus which provides a curing chamber for bonding a pre-cured tread strip to a tire casing, and in which the proper amount of pressure and heat is provided to effectuate the bond, without any likelihood of damage to the casing or destruction of the bond. In the apparatus to be described, in certain of its embodiments, pressurized steam is introduced into the curing chamber to supply the required temperature and pressure for permanently bonding the pre-cured tread to the tire casing; and pressurized air or steam pressure is introduced into the casing itself to offset the steam pressure in the chamber so as to prevent the tire from buckling or collapsing.
In accordance with the aforesaid embodiments of the invention, the pressure differential between the internal and external pressures is automatically maintained at a pre-set value throughout the curing process, and excessive pressure differentials are prevented. As noted, such excessive pressure differentials not only could destroy the bond between the tread and tire casing, but could also result in permanent damage to the casing itself.
In general, the apparatus of the invention provides an automatic regulator control system for maintaining the pressure differential between the pressurized fluid outside the casing and the pressurized fluid inside the casing at a pre-set value throughout a bonding or vulcanizing process, so as to preclude any possibility of an excessive pressure differential being established which not only could result in preventing a good bond between the tread and the casing, but could result in damage to the tread or to the casing itself.